Implantable stimulation devices generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Patent Publication 2007/0038250.
Spinal cord stimulation is a well-accepted clinical method for reducing pain in certain populations of patients. An SCS system typically includes an Implantable Pulse Generator (IPG), electrodes, at least one electrode lead, and, optionally, at least one electrode lead extension. As shown in FIG. 1, the electrodes 106, which reside on a distal end of the electrode lead 102, are typically implanted along the dura 102 of the spinal cord 104, and the IPG 100 generates electrical pulses that are delivered through the electrodes 106 to the nerve fibers within the spinal column 104. Electrodes 106 are arranged in a desired pattern and spacing to create an electrode array 110. Individual wires 112 within one or more electrode leads 102 connect with each electrode 106 in the array 110. The electrode lead(s) 102 exit the spinal column 104 and may attach to one or more electrode lead extensions 119a and 119b. The electrode lead extensions 119a and 119b, in turn, are typically tunneled around the torso of the patient to a subcutaneous pocket where the IPG 100 is implanted. Alternatively, the electrode lead 102 may directly connect with the IPG 100.
As should be obvious, an IPG needs electrical power to function. Such power can be provided in several different ways, such as through the use of a rechargeable or non-rechargeable battery or through electromagnetic (EM) induction provided from an external charger, or from combinations of these and other approaches, which are discussed in further detail in U.S. Pat. No. 6,553,263. Perhaps the favorite of these approaches is to use a rechargeable battery in the IPG, such as a Lithium-ion battery or a Lithium-ion polymer battery. Such a rechargeable battery can generally supply sufficient power to run an IPG for a sufficient period (e.g., a day or more) between recharging. Recharging can occur through the use of EM induction, in which EM fields are sent by an external charger to the IPG. Thus, when the battery in the IPG needs recharging, the patient in which the IPG is implanted can activate the external charger to transcutaneously (i.e., through the patient's flesh 114) charge the battery (e.g., at night when the patient is sleeping or during other convenient periods). In FIG. 1A, the external charger is represented generically by coil 108, which coil can be used to produce an EM field 110 capable of transcutaneous transmission through the patient's flesh 114.
Several basic varieties of external charger designs possessing a charging coil (such as coil 108) have been disclosed in the prior art. See, e.g., U.S. Patent Publication 2009/0118796; U.S. Patent Publication 2010/0204756; and U.S. Patent Publication 2008/027500. The operation of these prior art external chargers function essentially as shown in FIG. 2. As shown, the system comprises, in relevant part, the external charger 158 and IPG 100. A primary coil 108 in the charger 158 produces an EM field 110 capable of transcutaneous transmission through a patient's flesh 114. The EM field 110 is met at the IPG 100 by another coil 200, and accordingly, an AC voltage is induced in that secondary coil 200. This AC voltage in turn is rectified to a DC voltage at a rectifier 202, which may comprise a standard bridge circuit. (There may additionally be data telemetry associated with the EM field 110, but this detail is ignored as impertinent to the present disclosure). The rectified DC voltage is, in turn, sent to a charge controller and protection circuit 204, which operates generally to regulate the DC voltage and to produce either a constant voltage or constant current, Ibat, output as necessary for recharging the IPG 100's internal rechargeable battery 206. Further details concerning external chargers can be found in the '955 application.
As shown in FIG. 3, electrical current flowing into the page at the lower end of coil 108 and out of the page at the upper end of coil 108 induces a magnetic field 110 having a prominent portion in a direction perpendicular to the plane in which the primary coil 108 lies. Primary coil 108 is typically formed of many turns of copper Litz wire, of which only a handful of individual turns are shown in FIG. 3 for clarity. Thus, when a face of the case of the external charger 158 is oriented in close proximity to an implanted device, such that the primary coil 108 is parallel to a corresponding secondary coil 200 within the IPG 100, the magnetic field generated by the primary coil 108 induces an electrical current within corresponding coil 200 to charge the battery 214 within, or otherwise provide power, to the IPG 100.
As shown in FIG. 3, the magnetic field generated by an unshielded primary coil generates a magnetic field which is in part directed toward the secondary coil where it performs useful work, and which is in part directed away from the secondary coil where the magnetic field energy is substantially wasted. If a higher percentage of the magnetic field from the primary coil could be directed to the implanted secondary coil, the energy required to drive the external charger could be reduced, which could allow the external charger to be made smaller. One such method of directing a higher percentage of the magnetic field from the primary coil towards the body is to use a magnetic field shield behind the primary coil's windings, such as is illustrated in U.S. Pat. No. 6,389,318. Such a design can enhance the energy transferring efficiency of the external charger/implantable device system by reflecting magnetic field lines back inwards. The magnetic field shield can be constructed of any material with a high permeability, such as, but not limited to, ferrite powder or ferrite plates.
Heretofore, attempts at producing a wireless and integrated (i.e., containing a power source, charging coil, and associated charging and/or telemetry circuitry in a single, self-contained package), compact, and low-profile external charger were complicated by the generation of excessive heating and eddy currents in the casing of the external charger's power source, usually a rechargeable battery. In the state of the art charging device, the external charger's battery is placed near the charging coil inside the charging device. Due to this close proximity, the magnetic field produced by the charge coil induces eddy current heating in the battery case. This has the undesirable effect of both additional device heating as well as reduced charging efficiency. As much as 20% of the power transmitted by the charge coil is lost due to this coupling between the charge coil and the battery.
Given these shortcomings, the art of implantable devices would benefit from an improved wireless external charger design that is integrated, compact, and low-profile, that also comprises a magnetic shield. Such a charger would provide for: increased charging efficiency; faster charging rates; increased patient safety and comfort; lower power requirements; and a smaller form factor. This disclosure presents a solution to this problem, disclosing an external charger comprising: a housing; a coil within the housing; a rechargeable battery within the housing; and a magnetic shield within the housing comprising a plate or plates made of a high permeability material, wherein the magnetic shield is located between the battery and the coil, wherein the coil, battery, and magnetic shield are co-axially aligned, and wherein the coil is used to provide power to an implantable medical device.